Grindstone



FIG. 1 is a perspective view of a grindstone;

FIG. 2 is a front view thereof, the opposite side being an identical image of the side shown herein;

FIG. 3 is a left side view thereof, the opposite side being an identical image of the side shown herein; and,

FIG. 4 is a top view thereof.

The portion shown by broken lines is for illustrative purposes only and forms no part of the claimed design. The dashed lines show a borderline between the claimed design and other portion. 

The ornamental design for a grindstone, as shown and described. 